1° / IC/95/210
INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS
BROAD BAND SEISMOLOGY IN THE SCOTIA REGION. THE BASE ESPERANZA SEISMOLOGICAL OBSERVATORY
M. Russi INTERNATIONAL ATOMIC ENERGY G> Costa
AGENCY and
J. Febrer
UNITED NATIONS EDUCATIONAL, SCIENTIFIC AND CULTURAL ORGANIZATION M.RAMARE-TR.ESTE
VOL 2 7ift 0 8 IC/95/210 ABSTRACT
International Atomic Energy Agency The lithospheric study and the identification of relevant lateral heterogeneities in the and Antarctic continent and borderlands, is essential to undersrand the geodynamic evolution United Nations Educational Scientific and Cultural Organization both of the continental and oceanic bordering regions. The complexity of the geological evolution and the structural properties of the litho- INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS sphere in the Scotia area have been stressed by many authors. The present setting of the area is the result of the mutual interaction among the Antarctic, South American and several minor plates whose geodynamic history and actual boundaries are stiil partially unknown. The intense seismic activity that characterises the region encourages the use of the seismological approach to investigate the lithospheric structure of the area. BROAD BAND SEISMOLOGY IN THE SCOTIA REGION. Since January 1992 a broad band three components ssta? ion is operating at the Antarc- THE BASE ESPERANZA SEISMOLOGICAL OBSERVATORY tic base Esperanza in the NE area of Antarctic Peninsula. The station has been installed with the financial support of the Italian Programma Nazi'male di Ricerche in Antartide (PNRA) by Osservatorio Geofisico Sperimentale (OGS) and Instituto Antartico Argentino (IAA). Russi et al. (1994) have analysed selected recordings using the frequency-time analysis M. Russi (FTAN) method obtaining some relevant information on i he largo scale structure of the Osservatorio Geotisico Sperimentale di Trieste, lithosphere in the Scotia region even if data recorded by i single station were available. P.O. Box 2011, 34016 Trieste, Italy, The extension of our analysis to further events and to horizontal component records is here presented. G. Costa Within the framework of the international co-operation t o the Antarctic Seismographic International Centre for Theoretical Physics, Trieste, Italy Network, the OGS and the IAA are upgrading the Esper-uiza station and installing an and additional broad band station near the town of Ushuaia (Tierra del Fuego, Argentina) Istituto di Geodesia e Geofisica, Universita di Trieste, with the financial support of PNRA. Via dell'Universita 7, 34100 Trieste, Italy The inversion of the dispersion curves through the FTA N of the signals recorded by an increased number of stations and generated by events with source-station paths spanning and the region will allow us to extract the elastic and anelasti<- properties of the lithosphere J. Febrer in the area. Instituto Antartico Argentino, Cerrito 1248, (1010) BS.AS, Argentina.
MIRAMARE - TRIESTE August 1995 some of their families with children, live in the Base, while during summer about 60 people work in the Base due to the presence of the rese.irchers of IAA and guest scientists from other countries. The base consists of a group of 13 small red-coloured living houses, a radio station, Introduction. some maintenance shops and depositories, a small heliport and a main building with the base kitchen and refectory. There is also a little chapel and a school for children The study of the lithosphere and the identification of relevant lateral discontinuities which follow their parents in the one-year stay at Esperanza in the Antarctic conlinent and in the neighbouring areas, is essential to the As a matter of fact, this very particular organization makes the base very similar to understanding of the geodinamic evolution of the continental regions and of the a small village and is therefore completely different from what we usually expect to surrounding oceans. find in the majority of the permanent Antarctic bases. A remarkable role in this complicated puzzle is played by the Scotia region, an area The climate is rather mild with a medium temperature around 0 C° in January and where the major Antarctic and South American plates interact with several minor minimals around -3D C° during the coldest season. Winds are rather strong and can ones generating a variety of tectonic phenomena. reach speeds around 100 km/h during summer and more than 200 kni/h during winter. The most evident structural feature is the arc formed by the group of submarine ridges and volcanic islands closed to the East which connects the southernmost South America to the Antarctic Peninsula (Dalziel, 1984). Subduction phenomena characterize the Sandwich Is. area, which forms the eastern margin of this arc. while a Base Esperanza Seismographic Station (1992-1994). spreading centre is active more to the East between the Sandwich and Scotia Plates. The Shackleton transform fault and the yet poorly known South Shetland Is. area close Base Esperanza station (Fig. 1) has started operating on January 20, 1992 (Fanzutti the loop on the West side. The northern and southern margins are represented by the et al., 1992). The instrumentation originally employed consisted of three Teledyne North Scotia Ridge and the South Scotia Ridge whose character is basically Geotech BB-13 broad band seismometers connected to a PDAS-100 acquisition continental. An exhaustive description of the geodynamics of the Scotia region can be system. The equipment has been installed in the interior nf a small already existing found in Barker and Dalziel (1983). wooden building located in the base, where electric energy to power the The solution of the Scotia region geological puzzle can be found only in the joint instrumentation is available. interpretation of collected data using different geophysical methods. Seismology Prior to the installation of the instruments a square hole about 2m x 2m wide and might give substantial contribution due to the remarkable seismicity level of the whole 50 cm deep was excavated until the underlying rocky ground was reached and a area (Pelayo and Wiens, 1989) when a suitable data base is available. cement pillar was erected over it as a basement for the seismometers. The station geographic WGS 84 coordinates are:
Latitude: 63°•23" 53" South A short panorama of Base Esperanza Longitude: 56° 59' 47" W,;st Height: 31 m Bahia Esperanza (Hope Bay) is located at the end of the Antarctic Peninsula on the Antarctic Sound facing the icy profiles of Joinville, D'Urville and Dundee islands. To ensure a proper protection from excessive temperature variations the The base has been built in the very proximity of the sea at the Western side of seismometers were installed in the interior of a wooden cover. To reduce temperature Caleta Choza facing the rocky outcrops of Baliza Ciega and Rocas Dcnticuladas. gradients due to the cement basement also the floor hus been also covered with On the east side of the bay the gently sloped ice surface of Glaciar Buenos Aires insulating material, an exception being made for the small areas left unprotected approaches the sea, where it finally ends with a high ice scarp. Behind the base Mount where the feet of the seismometers ensure a proper coupling with the basement. Flora is located. About 600 meters from the base an old English settlement, named Temperature control has been provided by means of an energy source represented Trinity base and presently abandoned is found. by three 75 W lamps operating under the control of a simple thermostat. Power for The connection between the base and the airport of Rio Gallegos (Argentina) is the instrumentation has been obtained from a set of six 2 V batteries float charged by established more or less monthly by an Hercules C130 flying from Rio Gallegos to a battery charger connected to the main power source of the base. The timing for the Base Marambio to ensure supplies and people turnover to the Argentinian bases in the seismograph was originally obtained by means of the internal clock of the PDAS-100 Antarctic Peninsula. system. The flight from Rio Gallegos to Marambio lasts about three hours. The last jump, To protect the PDAS-100 Teledyne Geotech acquisition system, and to allow about 100 km wide, from Base Marambio to Base Esperanza is performed by confortable operation conditions for the operator charged with the station maintenance helicopters or Twin-Otter planes landing on the Buenos Aires Glacier in the proximity and data recovery, a small wooden room has been realized in the interior of the station of Base Esperanza. During winter about 30 people, technicians, military', including building to be used as an emergency laboratory. For each seismic channel two continuous data streams were recorded. For the The first step toward the new observatory establishment has been made during the primary one a rate of 2 samples per second was set, while for the secondary one a 0.2 Antarctic summer 1994, when a detailed field survey on the surroundings of Base samples per second acquisition rate was chosen. This values of the sampling rates was Esperanza has been done to select the final location for the instrumentation. suggested by the small amount (about 8 megabytes of ram memory) of storage Preliminary noise measurement were made using an ink recorder during January available, so that the data recovery had to be done every three days. and February 1994 and brought to the conclusion that the best suitable location would Data were transferred from the PDAS-100 memory to floppy disks by the IAA have been a site in the proximity of the abandoned Trinity Base settlement. operator which remains at Base Esperanza during a whole year. The operator also The digital noise recordings performed during the Antarctic summer 1995 survey performed the first processing of the data which consisted in the display on a PC confirmed the preliminary results, which showed comparable noise level conditions in computer screen of all the signals for their selection. Diskettes containing a copy of the actual location and in the proximity of Trinity Base, with the exception for the the original data and of the selected events were sent, more or less monthly, to the short-period noise caused by human activities in the base. IAA in Buenos Aires and to the OGS in Trieste. The settlement of the seismological observatory in the Trinity site implies the During the first year of operation the instrumentation worked satisfactorily even if construction of a new housing for the station equipment. Electrical energy is easily some problems arose due to the thermostatization system. available from the base which is about 600 m away. Access to the site for Improvements to the installation were performed during the 1992-'93 Antarctic maintenance and data retrieval does not imply major problems in any season. A summer. To improve the time base a GPS signal was derived from a digital telemetric connection between the seismograph and the ba>,e in order to allow remote seismograph equipped with a Mark L4-C short period geophone which had been connection is under study. installed beside the Italian seismograph by a scientist of the Museo Nacional de The hypothesis of a direct connection with the IAA in Buenos Aires and OGS in Ciencias Naturales (Madrid, Spain). The two seismographs worked one beside the Italy has been taken into consideration and some tests have been performed using the other for about two years allowing the recording of local, regional and teleseismic radio equipments of the base. To perform the experiment the digital waveforms of the events. The recording of the Italian seismograph alone were not optimal due to the January 17, 1951 LQS Angeles earthquake recorded by the Esperanza seismograph limitations in sampling. have been used. Also the thermostatization system has been modified using two included wooden A two-way transmission to and from Buenos Aires has been performed to check boxes and an oil filled heater in the intermediate volume and since then it has always the quality of the connection. The experiment was success! ul, but the transfer velocity behaved satisfactorily realizing optima! thermal conditions for recording at a allowed by the available radio equipment was so slow thai this method could be used temperature of 6 C°to 7 C°. only for exceptional situations. The possibility to use satellite links is actually under An automatic system for water pumping was added to avoid flooding of the consideration. building and possible damages to the instrumentation due to snow melting during In the meantime, a fundamental step in the station upgrading process has been warm days in the summer season. performed with the replacement of the PDAS-100- BB-13 seismograph with a Reftek In this configuration the station has worked satisfactorily during the whole 1993 72A-08 - Guralp CMG-3T broad-band seismograph system. and for most part of 1994.
Instruments specifications. Transfoming the Base Esperanza seismographic station in a seismological observatory. In this paragraph we will limit ourselves only to a very concise description of the PDAS-100 system and the BB-13 seismometers and will concetrate our attention on The particularly favorable position of Esperanza, together with the good results the characteristics of the new digital recorder Reftek 72A-08 and Guralp CMG-3T obtained in the analysis work performed on the data recorded during 1992 and 1993, broad band seismograph system. convinced the Italian PNRA of the opportunity of a complete renewal of the station The PDAS-100 Portable Data Acquisition System (PDAS) allows the acquisition instrumentation to overcome the limitations of the original system. of 3 channels (optionally 6) of analog data with programmable sampling rates from Consequently, it was decided to realize a permanent Seismological Observatory 0.1 sps up to 1000 sps. For each channel two independent (primary and secondary) equipped with broad band seismographs complying with the international standards output rates can be selected. Analog to digital conversion is performed with 16 bit accepted for stations which partecipate to the Federation of Digital Seismograph resolution which allows a maximum dynamic range of 132 dB using gain-ranging. Networks (FDSN)(Russi. 1994). The model that was used recorded data on nonvolatile CMOS Static Random Access This decision is in line with the guidelines contained in the recommendations of Memory which allowed the storage of about 8 megabytes of seismic data. the Scientific Committee on Antarctic Research (SCAR) Solid Earth Geophysics The sensors used were a BB-13 Teledyne Geotech triplet. Working Group, which has repeatedly encouraged (ref. ICSU, Scar Bulletin n.109, The poles (no zeros) April 1993) the continued operation of existing broadband seismograph stations on the Antarctic continent and the establishment of new ones in optimal locations. -7.063+17.271i -7.063-17.27 li -20.816 in Hz units, permit the calculation of the seismometer response curves, which show a Esperanza station and the Scotia region broad-band seismograph network flat response to acceleration from dc to 20 Hz. project. The new equipment, which has started operation on February 5, 1995 is based on the well-known seismometer Guralp CMG-3T coupled to a Reftek 72A-08 digital The intense seismicity characterizing the Scotia region is associated with recorder, and a GPS receiver supplies an accurate timing. subduction phenomena in the South Sandwich trench, where events of magnitude Ms The 72A-08 high-resolution recorder is a modular system, complying with 7 and more can take place, and with the transcurrencies of the North and South Scotia PASSCAL program specifications, which is basically formed of a main 72A-08 unit, a Ridges. A diffuse seismicity is evident also in the area we ,t of the Drake channel 72A-05 SCSI Disk recorder subsystem and a GPS Clock Model 111 A. A 1.3 (Fig. I). gigabytes 72A-03 DAT unit is used for data recovery. A PC computer, connected to The only possible way to build a suitable seismological daia base is represented by the 72A-08 via the communication port is used to load setup parameters and to control a regional broadband seismograph network to be operated in the area and integrated data recovery operations. within the Antarctic seismograph network. The recording unit employed at Esperanza can host 6 channels of 24 bit seismic After some contacts between the researchers of the two institutions, OGS and IAA data and up to 8 different data streams for each channel. Several types of recording prepared a project for the establishment of a network formed by five stations to be options and sampling rates are software selectable. We actually setup three distinct installed at sites located in Antarctica (Base Esperanza), Ticrra del Fuego (Ushuaia) data streams to record the three seismic components of the associated CMG-3T and on the islands distributed along Ihe margins of the Scotiii region. (Orcadas Base, seismometer. South Georgia Is., Sandwich Is.). Trigger recording with STA/LTA method is used for the streams sampled at 10 and Carrying out the complete project, which has obtained the approval of the National 50 sps, while continuous recording is used for the 1 sps data stream. Scientific Commission for Antartica of the PNRA, will require several years and This selection of the sampling rates and of the other parameters associated to each implies a strong cooperation with the institutions interested in the seismological study data stream should allow proper recordings of both regional and teleseismic events of the Western Antarctic area. and reasonable time intervals between each data backup, which is routinely performed Esperanza represents the first acquisition point of this network and the fundamental each 15 days. role it will play in the context of the Scotia region broad-band seismographic network The Gur.ilp CMG-3T seismometer response curves can be calculated from (he is already evident from the preceeding paragraphs. values of the transfer function poles A second station has already been financed by the PNRA and is being installed in the proximity of the town of Ushuaia (Tierra del Fuego, Aigentina). Also Ushuaia -7.07 * 10-V7.07i * 10"3 -7.07 * 10"3-7.07i * 10'-1 -80.5 + 3O.8i -80.5 - 30.8i station is equipped with a Reftek 72A-08 - Guralp CMG-3T digital seismograph. The data collected by Esperanza and Ushuaia stations will be extremely valuable to and zeroes improve the available information on the time and space distribution of the regional seismicity in the Drake Passage area, and in the Tierra del Fuego where there is very 0, 0, 150.5 poor information about local seismicity. To obtain the best possible results the existing IAA OGS cooperation has been extended to scientists operating in the United expressed in Hz. The CMG-3T resulting curves show a flat velocity response in the Kingdom and in the USA in order to put together also the sdsmological information band 0.01 Hz-50 Hz. of other broad band stations operated in the area. Among the main advantages of the new instrumentation we quote: As an example of local activity we mention a sequence of eaithquakes started in January 1994 in the proximity of South Shetland Is. at a distance of about 140 km - the much less restrictive conditions of temperature stability needed by the CMG- from Esperanza. A magnitude 5.1 earthquake was recorded on January 22 (epicenter 3T seismometer and its small dimensions compared with those of the BB-13 in the proximity of Bridgeman Is.). The Italian seismogiaph recorded this event seismometer; along with some aftershocks, but a lot of minor events were recorded by the short- - the easy conversion of data from the original Reftek acquisition formats to SEGY period instrument of the Spanish institute. and SAC standards; - the availability of adequate storage devices which allow the selection of proper signal sampling rate parameters. Crustal studies by means of waveform analysis using the Esperanza station dataset.
During the past four years (1992-1995) Esperanza station has recorded a conspicuous number of events with worldwide distribution of epicenters. Fig.2 shows a set of seismograms recorded by the new equipment on February 5,1995. The Esperanza station dataset has been used as a guideline for the definition of the References. methodologies to be applied in the data analysis. The main objective is the investigation of the structural properties of the lithosphere and the upper mantle in the Barker, P.F., Dalziel, I.W.D. (1983): Progress in geodynamics in the Scotia arc Scotia area by means of frequency time analysis (FTAN) methods and the study of the region. In R. Cabre (ed.), Geodynamics of Hie Eastern Pacific Region, seismic sources by means of waveform inversion techniques. Caribean and Scotia arcs., Geodynamics Series, American Geophysical The results obtained analyzing some events by means of FTAN are described in Union, Vol. 9, 137-170. Russi et al. (1994). The method has subsequently been applied to several events sampling the Scotia region to obtain a conspicuous number of dispersion relations Dalziel, I.W.D. (1984): The Scotia Arc: an international geological laboratory. from the recorded seismograms. Episodes, 7, 3, 8-13. In a preliminary work dispersion relations have been obtained from the vertical component recordings of events with epicenter mainly in the Sandwich Is. area. The Fanzutti, F., Febrer, J.M., Nieto, D.Y., Russi, M., 1992. Installazione di una analysis has been later extended to earthquakes whose great circle path sample the stazione sismica e una gravimetrica alia base argentina Esperanza, 1992.- Scotia region, and the horizontal components have been also processed. Programma nazionale di ricerche in Antartide. Rappono sulla campagna A preliminary step in the processing has been the conversion from the original Antartica Estate Australe 1991-92, ENEA.Roma. recording format to the formats required by the FTAN analysis package. The horizontal N-S and E-W components are rotated to provide the vertical and transversal International Council of Scientific Unions (ICSU) SCAR bulletin, n° 109 April components. Azimuth, backazimuth and epicentral distances are calculated. 1993. Recommendation SEG 1992-1 (revise from SEG 1990-1), page 13. Following these preliminary processing the frequency-time analysis is performed using the FTAN software described in Levshin et al. (1989,1992). Levshin, A.L., Yanovskaya, T.B., Lander, A.V., Bukchin, B.G., Barmin, M.P., In Figg. 3 to 6 a complete example of FTAN processing is shown. FTAN maps of Ratnikova, L.I. and Its, E.N., 1989: Seismic surface Waves in a Laterally the vertical, radial, and transverse components have been produced. By means of the Inhomogeneous Earth, (ed. Keilis-Borok, V.I.) Kluwer, Dordrecht. floating filtering technique the P and S phases and Rayleigh and Love wave phases have been extracted. Levshin, A.L., Ratnikova, L.I. and Berger, J., 1992: Peculiarities of surface wave Fig. 7 displays the original vertical, radial and transverse seismograms, and the propagation across Central Eurasia, Bull. Seism Soc. Am., 82,2464-2493. corresponding P, S, Rayleigh and Love phases as extracted by means of FTAN software. Pelayo, A.M. and Wiens, D. A. (1989): Seismotectonics and relative plate motions in Fig. 8 shows a set of dispersion Rayleigh curves calculated from Esperanza station the Scotia sea region. Jour. Geoph. Res., 94, B6, 7293-7320. recordings of Scotia region events (fig.9) compared with standard oceanic and continental curves. Russi, M., Febrer, .I.M., Costa, G., Nieto, D.Y., Panza, G.F., 1994. Analysis of The inversion of the surface wave dispersion curves, obtained through the FTAN digital waveforms recorded at the seismographic station Esperanza. Terra analysis of Esperanza and other properly located stations recordings, will allow the Antartica, vol. I n° 1, 162-166. reconstruction of the elastic and anelastic properties of the lithosphere in the Scotia area. Russi, M., 1994. A new permanent broad-band seismological Observatory at Base The structural models derived by the inversion of these curves will then be used to Esperanza. Antarctic Peninsula. January - February 1994 Site Survey. characterize the seismic sources in the area through the retrieval of the complete Italian Antarctic Research Programme, Earth Sciences, Field data reports IX seismic moment tensor and its time hystory by means of waveform inversion ItaliAntartide Expedition 1993-94, 75-78. techniques (e.g. Sileny et al.,1991; 1992). Sileny, J., and Panza, G.F., 1991. Inversion of seismograms to determine simultaneosly the moment tensor components and source time function for a point source buried in a horizontally layered medium. Studia geoph. geod. 35, 166-183.
Sileny, J., Panza G.F. and Campus P., 1992. Waveform inversion for point source moment tensor retrieval with optimization of hypocentral depth and structural model. Geophys. J. Int., 108,259-274. Figure captions.
Fig , Scotia region seismichy map: even, fro. .907 ,o .988 (USGS/NE1S G.oba. Hypocenter Data Base. Version 1.0). Dep.h >n meters. ary 5, .995 New Zealand event. Teleseism recorded by Esperanza Fig. 2 February station.
Fig 4 Frequency time analysis diagram for vertica. component of September 27. 1993 S.W. Atlantic Ocean event. P waves extraction.
FiE 5 Frequency time analysis diagram for vertica. component of September 27, g ,9 93 S W. Atlantic Ocean event. S waves extraction. Z
and Love waves obtained by FTAN analysis. Fig 8 Rayleigh dispersion curves for analyzed events compared with standard oceanic and continental dispersion curves.
Fig. 9 Great circle paths for analyzed events.
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